Systems for remote identification of objects are useful for many purposes, including identifying and locating objects such as trains or automobiles. Such systems use RF signals to communicate information between a reader apparatus and a remote transponder. The reader sends an RF signal to the transponder, an antenna in the transponder receives the signal, backscatter-modulates the received signal with data temporarily or permanently stored in the transponder (e.g., the identification number, the location of the transponder, or other information), and reflects this modulated signal back to the reader. The reader decodes these signals to obtain information from the transponder. The details of these transponders and readers have been previously described in U.S. Pat. Nos. 4,739,328 and 5,055,659, which are hereby incorporated by reference.
Because these transponders and readers communicate using microwave RF signals, it is difficult to restrict the zone in which a signal from a transponder can be read. However, in some reader-transponder systems, it is especially important that a reader not be able to transact with a transponder outside of a prescribed zone. In railroad applications, for example, readers may be mounted on locomotives and transponders positioned between the rails of track to report the locomotive's position and identify the track on which it is traveling. The information obtained by the reader is used as input for a train management system that can monitor and control the movement of numerous locomotives. Obviously, it is very important in such a system that the reader on a locomotive traveling on one track not read a transponder on an adjacent track. Such erroneous cross-track reading could lead to unnecessary braking (as when a locomotive reads the transponder on an adjacent track where another locomotive is traveling in the opposite direction) or worse (when two locomotives do not realize they are on the same track).
Train-mounted readers and track-mounted transponders using low-frequency magnetic coupling have been known for some time. In a typical system a transmitting coil at the reader sends power, and sometimes clock, information, or command signals, to a receiving coil at the transponder. If the transponder sends data back to the reader also via magnetic coupling, the inherent short range of the low-frequency magnetically coupled coils makes it essentially impossible for the reader to communicate with a transponder on an adjacent track. However, because the power transmission frequency or magnetic coupling retransmission frequency is usually in the range of 50 to 500 kHz, achievable data rates are limited to a few kilobits per second. Therefore, the data message is concomitantly limited to a few tens of bits at high train speeds.
Systems that power the transponder magnetically but return data from the transponder via a higher frequency (e.g., 915 MHz or 2450 MHz) RF transmitter or modulated RF reflector can have higher data rates and send longer messages in the available transaction time. But these systems run the risk that the transponder's RF signal will be captured and read by a reader on an adjacent track. RF systems must provide enough RF signal power to overcome increased RF attenuation due to such factors as snow, ice, water or debris covering the transponder, or degradation of reader and transponder performance with time and environmental effects. In a typical system, for example, a reserve of 30 db might be provided to accommodate the attenuation due to debris over a transponder. A link with sufficient reserve to read a transponder under worst-case conditions might, under favorable conditions, have sufficient capability for reading an RF signal from a transponder between the rails of an adjacent track. In addition, the presence of "stray" RF paths due to reflections off metal surfaces cannot be controlled in a practical way, especially in the cluttered environment underneath railway vehicles. Thus, the range of an ordinary RF data link cannot be restricted well enough to rule out these cross-track readings, which may be disastrous.